Included in the background hereof are the teachings of U.S. Pat. No. 9,478,942 of M. Feng, N. Holonyak, Jr., and M. K. Wu, assigned to the same assignee as the present Application, and incorporated herein by reference. Reference can also be made to documents cited therein, including: U.S. Pat. Nos. 7,091,082, 7,286,583, 7,354,780, 7,535,034, 7,693,195, 7,696,536, 7,711,015, 7,813,396, 7,888,199, 7,888,625, 7,953,133, 7,998,807, 8,005,124, 8,179,937, 8,179,939, 8,494,375, and 8,509,274; U.S. Patent Application Publication Numbers US2005/0040432, US2005/0054172, US2008/0240173, US2009/0134939, US2010/0034228, US2010/0202483, US2010/0202484, US2010/0272140, US2010/0289427, US2011/0150487, and US2012/0068151; and to PCT International Patent Publication Numbers WO/2005/020287 and WO/2006/093883 as well as to the publications referenced in U.S. Patent Application Publication Number US2012/0068151.
Bistability occurs in electrical or optical systems in which there is a region where the output signal has two stable energy states for a given input. Switching between these states can be achieved by a change of input level. The input-output relation forms a hysteresis loop, thus giving the bistability. Electrical bistable devices are fundamental to digital electronics as building blocks of switches, logic gates and memories in current computer systems. For example, any arrangement of transistors (such as CMOS or BJT) achieving two distinct stable states can be used as a storage element of a static random-access memory (SRAM) cell.
Today, digital electronic computers are bandwidth limited by the signal delay of RC time constants and carrier transit times of electronic logic. To overcome these problems, optical digital computers have been considered. Optics are capable of communicating high bandwidth channels in parallel without suffering interference. Similarly, optical bistable devices are fundamental to digital photonics as building blocks of optical switches, optical logic gates and optical memories. Two features are required to realize an optical bistable device: nonlinearity and feedback (see, for example, G. J. Lasher, Solid-State Electron. 7, 707 (1964); K. H. Levin and C. L. Tang, Appl. Phys. Lett. 34, 376 (1979); H. M. Gibbs, T. N. C. Venkatesan, S. L. McCall, A. Passner, A. C. Gossard, and W. Wiegmann, App. Phys. Lett. 34, 511 (1979); D. A. B. Miller, D. S. Chemla, T. C. Damen, T. H. Wood, C. A. Burns, A. C. Gossard, and W. Wiegmann, IEEE Journal of Quantum Electronics, 21, no. 2, 1462 (1985); B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics (2nd edition), Wiley, Section 23.4 Optical Gates.) An optical bistable system can be realized by the use of a nonlinear optical element whose output beam is used in a feedback system to control the transmission of light through the element. However, the full application of optics has yet to be applied to digital computers for reasons including the lack of suitable optical logic processors with scalable size and speed.
In 1991, semiconductor bistable photonic devices were reported based on the monolithic integration of a vertical cavity surface emitting laser (VCSEL) and a latching PNPN photothyristor (see P. Zhou, J. Cheng, C. F. Schyaus, S. Z. Sun, C. Hains, K. Zheng, E. Armour, W. Hsin, D. R. Myers, and G. A. Vawter, IEEE Photonics Technol. Lett. 3, pp. 1009-1012 (1991)) as well as two PNPN photothyristors (see W. K. Choi and Y. W. Choi, Electronics Lett. 43, No. 12 June 7 (2007). However, the major issue with a laser-photothyristor pair is that the PNPN-thyristor stores charge and has a very slow switching speed, typically in the MHz range. This fundamental limitation is owing to the saturated nature of the PNPN switching operation. Once turned on, the PNPN device accumulates large quantities of charge in its base, and takes a long time to turn off. This sets a fundamental limit to the speed of the laser-photothyristor to MHz switching. Other approaches based on external optical components such as semiconductor optical amplifiers (SOA), electro-absorption modulators (EAM) and Mach-Zehnder modulators (MZM) are limited by low coupling efficiencies and low extinction ratios. Furthermore, these components are usually built with large lateral dimensions for ease of optical coupling, and long lengths to increase the extinction ratios. Such difficulties and large device dimensions (˜mm) are difficult for achieving high density integrated designs as required for logic applications.
Starting about 2004, with quantum-wells (QWs) incorporated near the collector in the of a III-V heterojunction bipolar transistor (HBT), the radiative spontaneous recombination lifetime (τsp) of the device was reduced to a few picosecond (see e.g. M. Feng, N. Holonyak, Jr., and R. Chan, Appl. Phys. Lett. 84, 1952 (2004); H. W. Then, M. Feng, N. Holonyak, Jr., and C. H. Wu, Appl. Phys. Lett. 91, 033505 (2007). As a result, QW-HBTs with short base-collector metal contacts as in a two-terminal LED had demonstrated a record LED modulation bandwidth of f−3dB˜7 GHz and confirm a fast τsp˜23 ps operated at room temperature (G. Walter, C. H. Wu, H. W. Then, M. Feng, and N. Holonyak, Jr., Appl. Phys. Lett. 94, 231125 (2009)). Furthermore, the incorporation of an optical cavity for higher Q enclosing the QW-HBT provided higher photon density field-enhancement stimulated recombination, coherent light output, and invention of the transistor laser (see e.g. G. Walter, N. Holonyak, Jr., M. Feng, and R. Chan, Appl. Phys. Lett. 85, 4768 (2004); R. Chan, M. Feng, N. Holonyak, Jr., and G. Walter, Appl. Phys. Lett. 86, 131114 (2005); M. Feng, N. Holonyak, Jr., G. Walter, and R. Chan, Appl. Phys. Lett. 87, 131103, (2005)). The frequency response and resonance behavior of the semiconductor laser can be derived from the well-known Statz-deMars' coupled carrier-photon interaction rate equations (H. Statz and G. DeMars, Quantum Electronics, 530 (1960)). The modulation bandwidth is related to e-h radiation recombination lifetimes, photon lifetimes and cavity photon density. The transistor laser can thus improve modulation bandwidth and bit-error-rate owing to fast radiative recombination lifetimes determined by the thin base and ability of the transistor to inject and collect stored charge within picoseconds (forcing the base QW recombination to compete with E-C transport) (see M. Feng, H. W. Then, N. Holonyak, Jr., A. James, and G. Walter, Appl. Phys. Lett. 95, 033509 (2009); H. W. Then, M. Feng, and N. Holonyak, Jr., J. of Appl. Phys. 107, 094509 (2009); R. Bambery, C. Wang, F. Tan, M. Feng, and N. Holonyak, Jr., IEEE Photonics Technol. Lett. 27, no 6, 600 (2015).
Optical absorption for a direct-gap semiconductor can be enhanced in the presence of a static electrical field and has been explained as photon-assisted tunneling (PAT) in semiconductor surface (W. Franz, Z. Naturforsch. 13a, 484 (1958), L. V. Keldysh, Sov. Phys. JETP 34, 788 (1958); K. Tharmalingham, Phys. Rev. 130, 2204 (1963)) and used in a semiconductor PN junction diode (C. M. Wolfe, N. Holonyak, Jr., and G. E. Stillman, Physical Properties of Semiconductors, pp. 219-220, Prentice Hall, Englewood Cliffs, N.J. (1989)). However, previous studies have not included the effect of electro-optical cavity coupling and quality Q. In the transistor laser, the coherent photons generated at the base quantum-well interact with the collector field and “assist” optical cavity electron tunneling from the base valence band to the adjacent conduction band of the collector junction. As described in M. Feng, J. Qiu, C. Y. Wang, and N. Holonyak, Jr., J. Appl. Phys. 119, 084502 (2016), the optical absorption can be further enhanced by the cavity coherent photon intensity of the transistor laser.
The transistor laser intra-cavity photon-assisted tunneling (ICPAT) modulation via collector voltage (tunneling-collector voltage) is a unique property and the basis of ultrahigh speed direct laser voltage modulation and switching (see e.g. A. James, N. Holonyak, Jr., M. Feng, and G. Walter, IEEE Photonics Technol. Lett. 19, 680 (2007); M. K. Wu, M. Feng, and N. Holonyak, Jr., Appl. Phys. Lett. 101, 2010 (2012); M. Feng and N. Holonyak, Jr., Optics & Photonics News (OPN), Optical Society of America pp. 44-49 (OSA), March (2011); H. W. Then, M. Feng, and N. Holonyak, Jr., Proc. IEEE, 101, 2271 (2013).
It is among the objectives hereof to overcome limitations of existing approaches for achieving and exploiting bistability and fast switching in electro-optical circuits and techniques. It is also among the objects hereof to devise a new and improved method of operating a transistor device and modulating photon density in an optical cavity of the device.